1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to traction elevator systems having a direct current drive motor and a solid-state, dual-bridge converter power supply.
2. Description of the Prior Art
Solid-state, dual-bridge converters, which utilize controlled rectifiers or thyristors to interchange electrical energy between alternating and direct current circuits, are being used to supply adjustable, reversible direct current voltage and currents for the direct current drive motor of traction elevator systems. While solid-state converters have many advantages over the conventional motor-generator set as the direct current voltage source, they are also more sensitive to momentary interruptions and abnormalities in the alternating current voltage source. Such interruptions and abnormalities may cause a power fuse to blow, resulting in the elevator car being out of service until maintenance personnel replace the fuse.
Fuse blowing may also result if the wrong thyristor is inadvertently gated, a thyristor fails to commutate, or if one bank of the converter is activated while current is still flowing in the other bank.
To minimize fuse blowing, the prior art attempts to recognize those conditions which may result in fuse blowing, and upon detecting such a condition an orderly shutdown of the elevator system is initiated, such as by driving the active converter or bank into inversion end stop, suppressing gate drive, and causing the associated elevator car to make an emergency stop. The prior art arrangements still result in a certain amount of fuse blowing, and also in shutdowns to prevent fuse blowing which are actually unnecessary.
The prior art emergency stop arrangement drops the electromechanical brake, disconnects the motor armature from the direct current voltage source, and connects a dynamic braking resistor across the armature. The dynamic braking resistor is sized to prevent cable slippage on the drive sheave when the elevator car is moving downwardly at rated speed with rated load. If an emergency stop is made while the elevator car is moving upwardly at rated speed with rated load, the deceleration rate will be appreciably higher than for the downward travel direction. The relatively high deceleration rate may be reduced by reducing the dynamic braking force, i.e., by increasing the value of the dynamic braking resistor, but the deceleration rate for the fully loaded, downwardly moving elevator car will then be too low.
Thus, it would be desirable to more accurately discriminate between conditions where an emergency stop of the elevator car is actually required, and when it is not, in order to prevent unnecessary shutdowns. It would also be desirable to minimize fuse blowing due to inadvertent gating of the wrong thyristor, or activation of one converter bank while current is still flowing in the other bank. Finally, it would be desirable to reduce the deceleration range of the elevator car when an emergency stop is required.